The combustion of fossil fuels in activities such as electricity generation, transportation and manufacturing produces billions of tons of carbon dioxide annually. Research since the 1970s indicates increasing concentrations of carbon dioxide in the atmosphere may be responsible for altering the Earth's climate, changing the pH of the oceans and other potentially damaging effects. Countries around the world, including the United States, are seeking ways to mitigate emissions of carbon dioxide.
A mechanism for mitigating emissions is to convert carbon dioxide into economically valuable materials such as fuels and industrial chemicals. If the carbon dioxide is converted using energy from renewable sources, both mitigation of carbon dioxide emissions and conversion of renewable energy into a chemical form that can be stored for later use will be possible. Electrochemical and photochemical pathways are techniques for the carbon dioxide conversion.
Previous work in the field has many limitations, including the stability of systems used in the process, the efficiency of systems, the selectivity of the system or process for a desired chemical, the cost of materials used in systems/processes, the ability to control the process effectively, and the rate at which carbon dioxide is converted. Existing systems for producing synthesis gas rely on gasification of biomass or steam reformation of methane. The processes use high temperatures and pressures. In the case of synthesis gas made from fossil fuels, liquid fuels made therefrom increase greenhouse gas emissions. Synthesis gas from biomass can reduce greenhouse gas emissions, but can be difficult to convert efficiently and produces unwanted ash and other toxic substances. No commercially available solutions for converting carbon dioxide to economically valuable fuels or industrial chemicals currently exist. Laboratories around the world have attempted for many years to use electrochemistry and/or photochemistry to convert carbon dioxide to economically valuable products. Hundreds of publications exist on the subject, starting with work in the 19th century. Much of the work done prior to 1999 is summarized in “Greenhouse Gas Carbon Dioxide Mitigation Science and Technology”, by Halmann and Steinberg. A more recent overview of work on electrochemical means of reducing carbon dioxide is “Electrochemical Carbon Dioxide Reduction—Fundamental and Applied Topics (Review)”, by Maria Jitaru in Journal of the University of Chemical Technology and Metallurgy, 2007, pages 333-344.
Laboratory electrochemical methods usually involve a small (i.e., <1 liter) glass cell containing electrodes and an aqueous solution with supporting electrolyte in which carbon dioxide is bubbled, though a solvent other than water can be used. Reduction of the carbon dioxide takes place directly on the cathode or via a mediator in the solution that is either a transition metal or a transition metal complex. Photoelectrochemical methods also incorporate aqueous solutions with supporting electrolyte in which carbon dioxide is bubbled. The main difference is that some or all of the energy for reducing the carbon dioxide comes from sunlight. The reduction of the carbon dioxide takes place on a photovoltaic material, or on a catalyst photosensitized by a dye. All systems developed to date have failed to make commercial systems for the reasons outlined above. The systems developed in laboratories could not be scaled to commercial or industrial size because of various performance limitations.
Existing electrochemical and photochemical processes/systems have one or more of the following problems that prevent commercialization on a large scale. Several processes utilize metals such as ruthenium or gold that are rare and expensive. In other processes, organic solvents were used that made scaling the process difficult because of the costs and availability of the solvents, such as dimethyl sulfoxide, acetonitrile and propylene carbonate. Copper, silver and gold have been found to reduce carbon dioxide to various products. However, the electrodes are quickly “poisoned” by undesirable reactions on the electrode and often cease to work in less than an hour. Similarly, gallium-based semiconductors reduce carbon dioxide, but rapidly dissolve in water. Many cathodes make a mix of organic products. For instance, copper produces a mix of gases and liquids including methane, formic acid, ethylene and ethanol. A mix of products makes extraction and purification of the products costly and can result in undesirable waste products to dispose. Much of the work done to date on carbon dioxide reduction is inefficient because of high electrical potentials utilized, low faradaic yields of desired products and/or high pressure operation. The energy consumed for reducing carbon dioxide thus becomes prohibitive. Many conventional carbon dioxide reduction techniques have very low rates of reaction. For example, some commercial systems have current densities in excess of 100 milliamperes per centimeter squared (mA/cm2), while rates achieved in the laboratory are orders of magnitude less.